The present invention relates generally to the field of papermaking and, more specifically, to the retention of additives to the papermaking furnish or stock.
Paper and board are produced by forming a fiber mat from an aqueous cellulosic slurry on a wire screen. More specifically, as illustrated in FIG. 1, a papermaking system 10 includes a head box 11 which comprises a flow chamber located at an upstream end of a wire screen 12. The head box 11 receives a diluted cellulosic stock slurry referred to as a thin stock from a line 13 and deposits the thin stock slurry onto the wire screen 12. A paper or board mat is formed on the wire screen 12 and is transported out of the system at 14.
The thin stock that flows into the head box 11 typically includes less than 1% of solids, and therefore a large portion of the liquid is drained off the mat at the screen 12 and circulated through the line 15 to the silo shown at 16. This recirculated fluid is typically referred to as the white water and includes fines and fillers at a consistency typically between 0.05-0.5%.
In order to increase the strength of the final product especially in the recycled board production, large quantities of starch are added to the thin stock. The starch acts as a strength additive. Therefore, one of the important aspects of the papermaking process is the ability to retain the starch and other components added to the thin stock within the fiber mat that is formed on the wire screen 12.
Recirculation of the starch into the white water is inefficient for providing strength benefits and can cause a number of sanitation problems because the starch can break down into sugars and encourage the growth of bacteria in the system 10. Excessive amount of starch in the recycled white water also can increase the biological oxygen demand (BOD) in paper mill effluent. Residual starch in the white water can also increase the chemical oxygen demand (COD) or the amount of oxygen required to oxidize organic and oxidizable inorganic compounds in paper mill effluent. If a mill exceeds the government imposed BOD or COD limits, the mill can be subject to substantial fines.
Starch is generally provided in an uncooked and granular form and it must be gelled sometime during the papermaking process. Where large quantities of starch have to be applied often, paper manufacturers prefer using uncooked starch as opposed to cooked starch.
One method of improving the retention of granular starch in the fiber mat is the employment of a retention systems that employ coagulants and flocculants. Coagulants are low molecular weight cationic synthetic polymers or cationic starches that are added to the stock. Coagulants generally reduce the negative surface charges presented on granular starch particles and other particles in the stock and accomplish a degree of agglomeration of these particles.
Flocculants, on the other hand, are generally high molecular weight synthetic polymers operating via a bridging mechanism which creates larger agglomerates. The resulting agglomerates are then more easily entrapped in the formed web and therefore retention is improved. In contrast, unagglomorated small particles and starch granules would tend to pass through the web and back into the white water line 15.
While the use of flocculants enhances retention, they can have an adverse effect on formation. Formation is a measure of the uniformity of mass distribution within the paper sheet. A high variance of mass distribution is indicative of poor formation and therefore poor sheet or board quality. The use of retention aides such as coagulants and flocculant, while increasing retention, can also adversely affect formation. Hence, the need for a reasonable degree of formation is often a limiting factor in achieving higher levels of retention.
Therefore, there is a persisting need for an improved method of manufacturing of paper products with large quantities of starch which provides improved retention of the starch in the web while not adversely affecting formation of the mat or sheet.